Demetrios1453
Legend
Regarding D&D tradition.
Basic D&D officially kept religion off-camera, sotospeak. So normally, the topic of polytheism didnt even come up.
Old 0e D&D kept its polytheism in a separate splatbook, more like a booklet. Dont buy the splatbook, then no problem. It was separate from the three core booklets.
Likewise, 1e AD&D kept its polytheism in the separate splatbook, Deities & Demigods. The 1e Players Handbook had a picture of its Cleric as a monotheistic Christian (Catholic) priest wearing a crucifix, who was resurrecting a dead person.
The point is, formative D&D expected and encouraged players to create their own worlds. The official slogan was, "Your imagination is your only limit." If you want your world to have monotheism, then it has monotheism. If you want your world to have, polytheism then it has polytheism. If you want religion to be irrelevant, then it is irrelevant. The rules wanted you to do whatever you want. It rarely proscribed.
Homebrew is authentic D&D. The idea that there would be one "official" setting to rule them all, is anathema to original D&D tradition.
The increase in religious complexity over the course of D&D's history is simply due to the game becoming more complex. Everything was more simple back the start, when you only had four classes and a few races (which functioned as classes) to choose from, and none of those had any variants. More complex rules about clerical worship simply goes hand in hand with fighting-men having rangers and paladins branch off (and having their name revised in the process) and eventually acquiring archetypes (and a more complex rule set).
But, conversely, you do have a point in that the 5e PHB doesn't even mention non-polytheistic systems as an option, which is a bit strange. Looking at the 3.5e PHB, it still does assume polytheism as the default, but does give options for clerics who gain their powers from a cause or alignment, which opens the door to dualistic, monotheistic, or outright atheistic worlds and clerics within them. Obviously, it wouldn't be very hard to house-rule this for a non-standard campaign - clerics worship just one facet of the one true deity, or revere a myriad of saints of that deity, or worship causes or concepts in an agnostic or atheistic world - but again, this should have at least been brought up.
However, when it comes to this, it's not an "official" setting as such (as pretty much all settings in D&D are polytheistic), but more an "official" default for the game as a whole...